1. Although the coastal fog zone of the Namib Desert has negligible rainfall, large parts may be covered by soil-crust lichens with chlorophyll contents and photosynthetic rates (under optimal conditions of hydration and light), on an area basis, almost equal to the leaves of typical higher plants. 2. The photosynthetic and respiratory CO2 exchange rates of three soil-crust lichens, Acarospora cf. schleicheri, Caloplaca volkii and Lecidella crystallina were measured in the field (September 1990). The three species responded in a similar fashion to changing environmental conditions. Nocturnal hydration, by fog and/or dew, activated dark respiration which was followed after sunrise by a short period of positive net photosynthesis that continued until metabolic inactivation occurred from desiccation. Light compensation point for CO2 exchange was relatively high (28-43 \mumol m-2 s-1 photon flux density), and apparent quantum yield was low. This most probably was due to light interception by non-photosynthetic pigments in the lichens' cortex. Maximal water uptake by the biological soil crusts after heavy fog was 0.49-0.73 mm (precipitation equivalent). The moisture compensation point, i.e. the minimum water content allowing positive net photosynthesis, was very low (0.13-0.26 mm). 3. Area-related, maximal, daily net photosynthetic carbon gain of the three species under natural conditions was 158-290 mg C m-2 day-1. A very rough, first estimate of the annual carbon balance of the soil-crust lichens (photosynthetic gam minus repiratory losses) was 16 g C m-2 year-1. Keywords: Biological crust, dew, fog.